The safe and sanitary disposal of waste is an ages old problem. Untreated waste, both in solid and liquid form, may contain any number of substances noxious to humans and the environment, including particulate solids, organic and inorganic compounds and pathogens.
It is desirable, therefore, to treat the waste before disposal. The treatment of the waste to destroy pathogens can be accomplished by a number of methods.
One method to treat waste to kill pathogens is to heat it to a high temperature for a period of time. Commonly known as pasteurization, this process neutralizes pathogens to a degree dependent upon the level of temperature and length of time that the waste is exposed to the elevated level.
Pasteurization, while effective to neutralize pathogens, may not reduce the odors emanating from the waste and may not reduce vector attractiveness. In the absence of reduction of vector attractiveness, vectors such as rats, mice and flies, will be attracted to the untreated waste. Vectors pose a health risk by themselves, as well as potentially spreading any pathogens present in the waste. Therefore, any treatment and subsequent disposal must reduce odors and attendant vector attractiveness factors.
One method to reduce vector attractiveness and also neutralize pathogens is by lime stabilization, which elevates the pH of the waste to a sufficient degree, for a sufficient period of time. This method is usually accomplished by the addition of an alkaline substance to the waste. Substances such as calcium oxide or calcium carbonate and compounds consisting of or containing them such as lime or quicklime, lime kiln dust, cement kiln dust, or dolomitic lime are commonly used for this process. The relatively low expense of sufficient quantities of these materials and their high alkalinity makes them well suited to the task.
In addition to the noxious components potentially present in and possible vector attraction to untreated waste, a further disposal problem is presented by the fact that untreated waste rarely is purely solid. Rather it usually has both solid and liquid components, with the solid component further potentially containing some degree of bound liquid, usually water. Thus the percent of liquid in the waste may be a sum of both the free liquid component as well as the liquid bound to the solid component. Due to the presence of both these components, waste may vary from a liquid type consistency and appearance to a thick solid consistency and appearance. The need to deal with this variety of phases complicates treatment and disposal. For example, if the waste has mostly a liquid type consistency, the majority of the free liquid portion of waste is separated out and dealt with through techniques known in the art. The remaining solid portion, or sludge, includes the remaining free liquid water, any bound water and the solid. That sludge, which may have a solids content of from 1 to 4%, then undergoes a further dewatering step by any of a number of processes known in the art. If the waste is of a more solid consistency, then the dewatering is usually done is a single process.
After the waste has been dewatered sufficiently, it is referred to as dewatered sludge, which may have a solids content of anywhere from approximately 10% to 60% with the remainder water. This dewatered sludge is difficult to handle. The varying solids content and percentage of water as bound and free give the sludge physical characteristics ranging from a viscous, colloidal liquid to a dry cake or clay.
The Environmental Protective Agency (EPA) has promulgated regulations for proper treatment and disposal of waste or sludge. To ensure neutralization of pathogens to what it deems an environmentally safe level, the EPA has currently imposed two statutorily defined levels of processes for disposal of waste: Process to Significantly Reduce Pathogens (PSRP); and Process to Further Reduce Pathogens (PFRP). The use of either or both PSRP and PFRP depends upon the use to which the treated waste is to be put. Currently, PFRP result in a greater degree of pathogen reduction and waste treated by PFRP has less restriction surrounding its disposal. Although PSRP and PFRP as currently promulgated in Appendix II to 40 CFR 257 are limited to a few named processes, it is possible to qualify a process for either level by meeting, inter alia, the statutorily defined reduction in pathogens.
U.S. Pat. No. 4,781,842 discloses such an invention. Although the process set forth therein is not named as a PSRP or PFRP specifically in Appendix II 40 CFR 257, the process achieves pathogen reduction to current PFRP mandated levels. It does so by mixing the waste with lime or a lime mixture sufficient to raise the pH to 12 for at least a day, and then drying the lime waste mixture, by a natural or aeration process, for a period of time sufficient to reduce pathogens to the current PFRP regulations set forth in that patent.
The disclosure in the '842 patent is limited to current levels of pathogen reduction necessary to achieve PFRP, however. Changing regulations may lead to changing levels of pathogen reduction and the '842 patent does not seem to be easily adaptable to such a circumstance.
Accordingly, while there are many different methods that are used to stabilize or reduce pathogens in sludge and to condition sludge for reuse, including digestion (aerobic and anaerobic), lime stabilization, chlorine stabilization and composting, and while there are other processes that are used to reduce the volume and to stabilize the sludge, such as heat drying or incineration, such techniques may lend themselves to other problems.
For example, incinerators heat and burn sludge resulting in an ash which has no significant beneficial reuse potential. Heat drying treats the sludge in order to drive off the water contained within the sludge while leaving much organic or inorganic solids intact, with the end product usually being pelletized and used for fertilizer.
The methods used to heat sludges are generally broken down into two categories, direct heating and indirect heating. For direct heating, heat transfer is accomplished by direct contact between the heating medium and the wet sludge, with the heating medium normally being hot air or hot gases.
Indirect heating is accomplished by contact of wet solids with hot surfaces, with the heating medium being normally kept away from the sludge and heating a surface which is in contact with the sludge. The heating medium in such instances is usually hot water, steam, or hot oil.
The amount of heat which can be transferred through indirect methods is a function of the heat transferability of the heat medium, heat transfer surface, and the material to be heated. The amount of heat transferred is formulated as follows: EQU .sup.q cond=.sup.h cond.sup.A (.sup.t m-.sup.t s)
where:
.sup.q cond=conductive heat transfer, Btu per hour (kJ/hr); PA1 .sup.h cond=conductive heat transfer coefficient, Btu per hour per .degree. F. (kJ/hr/.degree. C.); PA1 A =area of heat transfer surface, square feet (m.sup.2); PA1 .sup.t m=temperature of heating medium--for example, steam, .degree. F. (.degree. C.) PA1 .sup.t s=temperature of sludge at drying surface, .degree. F. (.degree. C.).
Various methods have been developed to try to optimize total heat transfer. Most of these prior efforts have attempted to increase the surface area in order to increase the total heat transfer.
Prior inventions have often attempted to increase heat transfer primarily by increasing the surface area. Such techniques may, for example, include a hollow rotating shaft in order to move the heating medium through a larger surface area, thereby providing an increase in total heat transfer. Variations and improvements on such have included the addition of hollow disks on the rotating shafts, in order to get the heating medium to the exterior of the shaft.
The use of hot water as a medium is restricted to 212.degree. F. (100.degree. C.). At 212.degree. F., the temperature differential (.sup.t m-.sup.t s) is very small. As a result, total heat transfer is extremely low. To overcome this restriction, steam may be used to allow for higher heat medium temperatures. Alternatively, oil can also be used. Above 250.degree. F., steam develops pressures in excess of 15 psig. At this point, special constructions such as A.S.M.E. coding of all fabrication must be done for proper safety. In addition, once pressure is utilized, leaks can occur which allow the heating medium to escape into the sludge. Leaks cause not only loss of pressure and heat, but also contaminate the sludge or material being heated.
This invention teaches a method and apparatus which increases the temperature differential (.sup.t m-.sup.t s) in order to increase the total heat transfer. The invention provides for an economical, high temperature method of heating the rotating mechanism to indirectly heat and to mix sludges with other material.
This invention thus provides a means to eliminate a fluid heat medium, avoiding elevated pressures and the possibility of leakage and contamination.
Accordingly, it is an object of the present invention to provide an apparatus and method to achieve currently mandated levels of PSRP and PFRP.
It is a further object of the present invention to provide an apparatus and method to achieve different levels than current regulations mandate of pathogen reduction in waste.
It is a further object of the present invention to provide a method and apparatus to achieve effective neutralization of pathogens in waste.
A further object is to provide an apparatus capable of both stabilizing and pasteurizing raw sludge in a low cost, time-efficient manner.
It is thus a further object of this invention to accomplish the above objects, wherein a method and apparatus is provided for adding supplemental heat to the sludge which indirectly heats the sludge by heat generated through electric elements.
Further objects and advantages, such as sensing the temperature of the sludge and controlling the amount of heat via a feedback control, by the placement of heat elements within either the mixing member, such as within a rotating cylindrical member, or by placing the heating elements outside or inside the walls of a chamber, are also provided.